The accusation comes from sources that are opposed to Isis as well as pro-Isis ones.

White phosphorus causes potentially fatal chemical burns which have been known to melt skin off the bone. It is similar to napalm.

Footage showed the signature spread of airburst white phosphorus munitions—probably M825 series 155mm artillery rounds—exploding over eastern Raqqa. It was posted last week by the anti-Isis activist group Raqqa is Being Slaughtered Silently.

When M825 rounds explode, they jettison roughly 115 felt wedges that are impregnated with white phosphorus.

If exploded high above the ground, the wedges can spread over a greater distance, starting fires over a wide area.

Colonel Ryan Dillon, a spokesman for the US-led coalition, told The Washington Post that the US military uses white phosphorus in “accordance with the law of armed conflict”.

The Pentagon posted photographs of Marine M777 howitzers in Syria—deployed to support the Raqqa operation—with a pallet of white phosphorus munitions in May.

The image was taken in March, and while the unit in the photograph probably has returned to the US, its replacement is thought to be using similar munitions.

Weapons

The weapons used in Raqqa look similar to those used a week ago in Mosul.

In October last year Amnesty International, responding to claims that white phosphorus was being used in Mosul, warned against the dangers of deploying it in any form.

“If buried by soil or water, white phosphorus wedges can be temporarily extinguished,” it said. “But they spontaneously reignite if they are uncovered, presenting a

serious hazard for unsuspecting civilians who may accidentally uncover them as they walk through the affected area.”

It comes as no surprise if the US is widely using white phosphorous.

In November 2005 BBC News reported that an article published in the US Army magazine Field Artillery noted it had been used during the battle for Fallujah.

The article written by a captain, a first lieutenant, and a sergeant, said “WP [White Phosphorus] proved to be an effective and versatile munition.

“We used it for screening missions at two breeches and, later in the fight, as a potent psychological weapon against the insurgents in trench lines and spider holes where we could not get effects on them with HE [High Explosives].

“We fired ‘shake and bake’ missions at the insurgents, using WP to flush them out and HE to take them out.”